1. Field of the Invention
The present invention relates to transmission apparatuses used in radio communication systems and base station apparatuses using them.
2. Description of the Related Art
CDMA (Code Division Multiple Access) has been developed as a multiple access system used in next generation mobile communication systems. In this CDMA cellular system, it is necessary to make a cell search for establishing initial synchronization when a mobile station turns on power or for cell switching (handover) when traveling.
Regarding the cell search method in this CDMA cellular system, as described in xe2x80x9cFast Cell Search Algorithm using Long Code Masking in DS-CDMA Asynchronous Cellular Systemxe2x80x9d by Higuchi, Sawabashi, Adachi, et al., in TECHNICAL REPORT OF IEICE (RCS96-122, 1997-01), a method is proposed by which long codes of downlink control channels are masked and correlation detection is performed on these masked parts using short codes which are common to cells to detect the timing and type of the long codes.
According to-this system, the transmitting side (base station) code-multiplexes symbols spread using a short code (CSC) which is common to cells and symbols spread using a long code group identification short code (GIC), with the long code masked part and transmits them, and the receiving side (mobile station) detects the timings by the short code which is common to cells, then limits long code candidates to be searched for by a long code identification section using the long code group identification short codes and specifies the cell-specific long codes from among these long code candidates, achieving a fast cell search. The above two short codes (CSC and GIC) are codes for search, called search codes hereinafter.
Furthermore, the transmitting side is provided with a plurality of transmission antennas per sector, and if control channel signals are spread using different short codes and each transmitted in parallel from a plurality of antennas, the control channel signals become more resistant to fading variations (especially during slow movement) and shadowing due to a transmission diversity effect, improving their reception characteristics.
In general, during parallel transmission, a same long code is used for a plurality of antennas and their transmission power is determined taking account of interference with other channels or other cells as follows: if transmission is performed with power of 1 through one antenna, then through two antennas transmission is performed with power of 0.5 each. At this time, the transmission characteristic of each antenna deteriorates by the degree transmission power is weakened, but a diversity effect produced when the receiving side combines a plurality of transmission signals improves reception characteristics in the long run.
However, in the cell search method above, if a same search code is transmitted from a plurality of antennas simultaneously, the number of matched filters will not increase on the receiving side but reception characteristics deteriorate due to independent fading variations. On the other hand, using different short codes (search codes) like control channels will cause problems such as code shortage or deterioration of reception characteristics due to an increase of the number of matched filters or increase of interference (mutual correlation between search codes) on the receiving side.
It is an objective of the present invention to provide a transmission apparatus and base station apparatus using it, capable of preventing deterioration of reception characteristics during a cell search on the receiving side due to fading variations or an increase of the number of matched filters and interference even when the transmitting side transmits control channel signals in parallel from a plurality of antennas.
A technology of transmitting signals in parallel from a plurality of antennas (parallel transmission) is being studied in radio communication systems. Suppose this parallel transmission includes all transmission systems simply carrying out parallel transmission from a plurality of antennas without depending on the transmitting order or timing of transmission signals or spreading codes. Recently, introducing an orthogonal transmission diversity system (OTD) using a plurality of antennas in CDMA radio communication systems is under study. This technology is intended to effectively improve reception characteristics through transmission diversity effects.
Thus, the present inventor, et al. came up with the present invention by taking notice of introducing a parallel transmission technology to cell searches in the CDMA cellular system and finding that its transmission diversity effect improves reception characteristics. This allows transmitted/received signals to become more resistant to fading variations (especially during slow movement) or shadowing, improving reception characteristics.
The present inventor, et al. also came up with the present invention by finding that applying the OTD technology to control channels, for example, perch channels, would improve reception characteristics through diversity effects. This makes it possible, to extend the cell radius (coverd area) while using same transmission power and suppress interference with other channels due to a reduction of perch channel transmission power in the same area.
That is, the key point of the present invention is TSDT (Time-Switched Transmit Diversity) which means when carrying out diversity transmission, control channels (CCH) are transmitted in parallel through a plurality of antennas with search codes inserted and by switching those antennas according to the search codes.
More specifically, antennas transmitting search codes are switched so that a search code may be transmitted through only one antenna in a given instant. In this case, switching of antennas can be performed either periodically or randomly. In other words, it is only search codes that are transmitted using TSTD. When a plurality of search codes are multiplexed and transmitted, either they may always be multiplexed and sent from a same antenna or they may be transmitted from different antennas. Transmission using a same antenna is necessary when using one code to detect another data-modulated code.